Relatively lockable elements, and particularly platforms, tables and the like, are used frequently in the furniture industry and such raising and locking platforms or tables are used especially in connection with sewing machine tables. The referenced application Ser. No. 101,826, filed Sept. 23, 1987, SCHMELLER, the disclosure of which is hereby incorporated by reference, is directed to a typical application for the locking mechanism of the present application. Raising tables, surfaces or platforms may also be used in other applications, for example in kitchens, workplaces and the like, where depressed platforms can be used to retain appliances, such as bread cutting machines, food mixers and the like; or small tools, such as grinders, which are mounted on a platform. The platform can be lowered to an inoperative position, and a cover plate then placed thereover, to have a flat table. The mechanism to shift the platform on which the appliance is located usually includes a locking mechanism which is so constructed that the platform is locked in a predetermined position. Locking and unlocking the platform will depend on the type of locking mechanism used. For example lock mechanisms and positioning mechanisms which use springs to compensate the weight of the appliance and of the platform. Basically, two systems are in use:
(1) Mechanism with over-compensation: The spring is strong enough to raise the platform and the appliance thereon into an operating position. To lower the appliance, for example below a work or table surface, the appliance and the platform are depressed to overcome the spring force. This depression is carried out until the lock mechanism latches and the platform is then retained in a lower position.
(2) Undercompensation: The mechanism must provide a stop which limits the lower position of the platform, and further a latch at the upper position which prevents depression of the platform from the raised position.
Under conditions of overcompensation, the platform is raised by spring pressure to an upper stop. In the lowered position, when it has been depressed manually, it is held by the latch or locking mechanism. If it is desired to raise the platform, it is depressed slightly, further, against spring pressure, which releases the locking mechanism and, by spring and manual force, the platform is lifted into the operative position against the upper stop. Normally, the upper stop can be made adjustable, so that the working level of the platform can be matched to the requirements of the user.
In undercompensated platforms, a latch is necessary close to the upper edge in order to prevent dropping of the platform from the operating position. If it is desired to lower the platform, then, it is necessary to first slightly raise the platform from the operating positon in order to unlock the locking or latch mechanism, so that the platform can then be lowered, for example together with an appliance or machine. The lowermost position is defined by a lower stop. Of course, the platform must be raised by lifting it with the appliance thereon. Springs can be used to assist the lifting movement but, as noted, with undercompensation, the spring is insufficiently strong to raise the platform without further assistance by an operator.
Various latch mechanisms are known; usually, they can define only a lower position and an upper position of the platform, are relatively complex, and subject to malfunction.